Question

A plant reaches its adult stage when _____.

Answer 1

In contrast to humans, who reach the adult stage when we're around 18 or around 21 and consider ourselves to be magically adults, plants have a much more naturally defined demarcation line where they become adults - this would be when they are able to reproduce. 

Answer 2

The correct answer is letter C. When plants are ready to reproduce they have reached their adult stage. This is when budding or flowering starts in plants and they reproduce through its spores. They get pollinated and eventually turn into fruits with seeds inside.